Pea Nutrient Requirements in Alberta

With the introduction of new pea cultivars that featured improved agronomic traits, Alberta pea acreage rose from less than 10,000 acres in the mid 1970's to more than 640,000 acres in 2000.

As acreage grew, so did the demand for information on the nutrient requirements for pea being grown in Alberta. Much of this interest focused on whether inoculant, nitrogen and phosphate fertilizers were needed for this crop under Alberta conditions.

Alberta Agriculture, Food and Rural Development led a study from 1995 to 1998 to find out what effects rhizobia inoculation, starter nitrogen (N), phosphorus (P), potassium (K) and sulfur (S) fertilizers had on the yield and protein content of four semi-leafless pea cultivars. Trials were conducted at 58 sites across Alberta over a wide range of agro-eco zones.

The study was done in partnership with Agriculture and Agri-Food Canada and Westco, with funding from the Alberta Agricultural Research Institute, Alberta Pulse Growers Commission, Agrium and Westco.

The study had four objectives:

to examine the responsiveness of different types of pea cultivars to rhizobium bacteria.

to determine the need for additional nitrogen fertilizer for optimum crop production.

Radley pea consistently had the lowest yield and the highest protein concentration.

Carneval and Majoret pea frequently had the highest yields, while Carneval pea had the lowest protein concentration.

Baroness pea produced yields similar to Carneval pea and Majoret pea, except in the Black soil zone, where it was lower.

Majoret pea consistently had the highest seed N yield (total nitrogen in the seed), which was 9 to 27 per cent higher than Radley pea. Radley had the lowest seed N yield simply due to lower yield.

A general rule of thumb is 50 bu/ac of peas requires 150 lb of nitrogen, 45 lb of phosphate, 140 lb of potassium, and 13 lb of sulfur. These requirements are supplied through soil, rhizobia and fertilizer.

Nitrogen (N) Fertilizer and Inoculant Response

To determine pea responsiveness to nitrogen and inoculant, 58 field trials using urea (46-0-0) were done over a wide range of soil types across Alberta. Four rates of 0, 18, 36 or 54 lb N/ac (0, 20, 40 or 60 kg N/ ha) were banded before seeding with and without pea seed inoculation. Table 1 shows the research site locations, mean yield and mean protein of all treatments, per cent yield and protein increases when using inoculant and nitrogen fertilizer.

The trials showed:

rhizobia inoculation increased pea seed yield in 41 per cent of trials across Alberta by an average of 14 per cent. Pea yield was unaffected by inoculation in the remaining trials.

in inoculated treatments on land with no history of legumes, the yield rose only slightly more often than on inoculated land that previously had a legume crop (45 per cent versus 38 per cent). However, the yield increase after inoculation was significantly larger (19 per cent vs. 5 per cent) on land with no history of legumes.

inoculation had a major effect on pea cultivars in 6 of 22 trials. Majoret pea responded the best to inoculation, while Baroness pea was the least responsive. From this study, it was estimated that approximately 80 per cent of N in the pea plant comes from nitrogen fixation.

application of N fertilizer improved pea yield in 24 per cent of the 58 trials conducted with an average increase of 9 per cent. When spring soil test N to 12 inches (30 cm) was less than 18 lb/ac (20 kg N/ha), application of N fertilizer increased pea yield in one third of the trials by an average of 11 per cent. The modest benefits of rhizobia inoculation or starter N fertilizer in most trials, despite appreciable deficits between spring soil test N and accumulation of N by pea, was likely due to adequate nodulation of pea by naturally occurring rhizobia.

generally, the effects of applied N fertilizer on protein concentration were small. Increases in protein concentration due to addition of N fertilizer were most frequent when soil test nitrate nitrogen (NO3-N) was less than 18 lb N/ac (20 kg N/ha). In trials that showed significant benefit from applied N fertilizer, the average protein concentration increased significantly with the rate of N application from a 0.9% increase at 18 lb N/ac (20 kg N/ha) to a 2.9% increase at 54 lb N/ac (60 kg N/ha)

Nitrogen fertilizer response information table (Table 2) was developed for pea growers from the research results. The following observations were also noted:

Pea benefits little from starter N. Starter N is only recommended when soil test N levels are less than 18 lb/ac in the 0 - 12 inch soil depth, in the Brown, Dark Brown and Thin Black Soil areas. Generally, nitrogen fertilizer is not required for pea grown on Black or Gray Wooded soils in central and northern Alberta. Frequently, most irrigated fields in southern Alberta have soil N levels greater than 40 lb N/ac and therefore do not require N.

If soil test N levels are less than 18 lb N/ac (20 kg N/ha), a small amount of N fertilizer should be either seed-placed or side banded at the time of seeding. N fertilizer could also be banded or broadcast and incorporated before seeding. Small amounts (< 20 lb N/ac) of urea (46-0-0) or ammonium nitrate (34-0-0) can be safely seed-placed, when seed bed moisture is good.

Soil tests are important when choosing whether to apply fertilizer. Generally speaking, with the exception of beans, little or no additional N fertilizer is needed by most pulse crops. In fact, excess N fertilizer may reduce the amount of N fixed by a pulse crop and delay crop maturity. Mid-season N applications are normally not recommended except when inoculation has failed and there is an obvious N deficiency.

Phosphorus (P) Response

Soil tests determine the amount of phosphorus (P) in the soil, and fertilizer recommendations are made in terms of phosphate (P2O5). Fertilizer is also sold on the basis of per cent P2O5 in the fertilizer product. To convert P to P2O5, multiply by 2.3 (e.g. 10 lb/ac of P = 23 lb/ac of P2O5). Commercial granular phosphate fertilizer is generally sold as monoammonium phosphate and contains 11 to 12 per cent nitrogen and 51 to 55 per cent phosphate (P2O5).

To determine pea responsiveness to phosphate fertilizer, 52 field trials using triple superphosphate (TSP; 0-45-0) were completed over a wide range of soil types across Alberta. Five rates of 0, 14, 27, 41 and 54 lb P2O5/ac (0, 15, 30, 45 and 60 kg P2O5/ha) were placed with the seed or band applied before seeding Carneval pea. Table 3 shows the research site locations, mean yield and mean protein of all treatments, per cent yield and protein increase from using phosphate fertilizer.

applying phosphate significantly increased pea seed yield at 19 of 52 sites. The average increase in seed yield from a phosphate application was 7 per cent.

the average yield benefit was similar in the Thin Black, Black and Gray Wooded soil zones, but was negligible in the Dark Brown soil zone and in irrigated trials.

of the 31 trials with soil test P (modified Kelowna method) levels of less than 27 lb P/ac (30 kg P/ha) to 6 inches (15 cm), 52 per cent had a significant yield increase due to the application of phosphate, while only 1 of 17 trials with soil test P levels of more than 27 lb P/ac (30 kg P/ha) had a significant yield increase. Application of 27 lb P2O5/ac (30 kg P2O5/ha) was enough to get close to maximum yields in trials with soil test P levels of less than 27 lb P/ac.

Phosphate placement had no effect on pea yields. There was no difference in pea yield between seed-placed and banded P. The minimal impact of seed placement on yield in this study was likely due to the less damaging effects to seedling growth of 0-45-0 than of the more commonly used source of P, monoammonium phosphate (MAP). Seed protein and P concentrations were not strongly affected by phosphate fertilizer application.

Phosphorus soil tests and fertilizer recommendations developed for pea in Alberta are fairly effective in predicting when a crop will respond to P fertilizer application. Soils that have accumulated fertilizer P over the years may still test deficient in plant-available P, particularly on high pH calcareous soils. Yet, crops grown on this land may not respond to added P fertilizer. This situation occurs most frequently in the Brown and Dark Brown soil areas of Alberta.

Table 4 shows phosphate fertilizer recommendations for pea in Alberta. P fertilizer does not have a strong effect on pea. Results from this project suggest that pea is most responsive to P fertilizer when soil P levels are less than 27 lb P/ac. Above this level, there is relatively little chance P fertilizer will have any effect. When soil test P levels are medium and significant P fertilizer was applied in the past 10 to 20 years, an annual maintenance application of phosphate fertilizer can be used to meet crop requirements and replenish soil P that is removed.

The availability of potassium (K) or sulfur (S) did not seem to be a significant factor in these trials. Adding K and S fertilizer increased yield significantly in only 3 of 44 trials. There did not seem to be a correlation between the response to K and S additions and soil test levels. However, recommendations were developed with general guidelines for when K and S fertilizers may be required.

Potassium (K)
The K soil test determines the amount of potassium in the soil, and fertilizer recommendations are made in terms of potassium oxide (K20). Fertilizer is sold on the basis of per cent K20 in the fertilizer product. The most common commercial potassium fertilizer sold is potassium chloride (KCl) and is often referred to as muriate of potash.

Pea takes up relatively large amounts of K. However, only 20 per cent of the K is in the seed, while the remaining K, contained in the leaves and stems, is returned to the soil. Many prairie soils have K levels in the range of 400 to over 1,000 lb/ac. Generally, K fertilizer is ineffective on pea when soil test levels are more than 300 lb/ac. K fertilizer may be needed on fields that test less than 300 lb/ac, or on sandy soils or intensively cropped fields.

Potassium is less mobile in soil than nitrate-nitrogen, but more mobile than phosphate fertilizer. Potassium fertilizers are more efficient when seed-placed or banded. However, even small amounts of seed-placed potassium with pea may reduce germination and emergence. Therefore, if potassium is required, banding may be the better placement method. Table 5 gives generalized K fertilizer recommendations for pea grown on various soil types in Alberta.

Sulfur (S)
Pea has a moderate requirement for S. Sulfur levels are generally adequate in the Brown and Dark Brown soil zones; however, some soil types are occasionally low in sulfur. Much of the sulfur in the topsoil is contained in the soil organic matter and is slowly released as sulfate-sulfur (SO4-S), the form a pea crop needs. Sulfate-sulfur is similar to nitrate-nitrogen in that it is very mobile in soil. Irrigation water contains substantial amounts of sulfate-sulfur. Although amounts in the water vary over time, 12 inches of irrigation water will add approximately 30 lb/ac of sulfate-sulfur into the soil. Therefore, pea crops rarely require S when grown on irrigated land.

Sulfate sulfur can be highly variable across fields. When a field is uniformly low in S, a soil test is very useful to estimate S fertilizer needs. However, if 10 to 20 per cent of a field is low in S, it can be difficult to identify these low S areas without intensive soil sampling.

Table 6 can be used as a guide to determine if sulfur fertilizer is required for growing pea in Alberta. If more than 10 to 15 per cent of a field is S deficient, it may be cost-effective to apply S fertilizer to the entire field.

Summary

given the uncertainty of naturally occurring levels of rhizobia in soil and the responsiveness of the many pea cultivars available to growers, rhizobia inoculation is strongly recommended. Rhizobia inoculation represents a cost-effective means of ensuring maximum pea yields. However, the frequency and magnitude of inoculation benefits are likely to be less than 50 per cent, especially in fields that have regularly grown a pea crop.

the benefits from applying starter N with pea are infrequent and usually small. Generally, starter N is only recommended when soil test N levels are less than 18 lb N /ac in the 0 to 12 inch soil depth in the Brown, Dark Brown and Thin Black soil areas of Alberta. Starter N is normally not recommended in the Black and Gray Wooded soil areas of Alberta. Nitrogen fertilizer recommendations are summarized in Table 2.

P fertilizer is not particularly effective on pea. Results from this project suggest that P fertilizer works best when soil P levels are <27 lb P/ac. Above this level, it is unlikely pea will respond to P fertilizer. Phosphate fertilizer recommendations are summarized in Table 4.

potassium and sulfur were not found to be major factors on pea yield. Adding K and S fertilizer resulted in significant yield increases in only 3 of 44 trials. No relationship was seen between the response to K and S additions and soil test levels. General recommendations and guidelines were developed for pea with K in Table 5 and S fertilizers in Table 6.